Surface temperature of Martian regolith with polygonal features: influence of the subsurface water ice

Kossacki KJ, Markiewicz WJ, Smith MD

Planetary and Space Science

51(9-10), 2003, 569-580, 10.1016/S0032-0633(03)00070-9

This work is an extension of our previous paper (Icarus 160 (2002) 73), where we analyzed seasonal cycle of condensation and sublimation of CO2 ice in the polygonal features. Such structures are visible in several images of the southern polar region, obtained by Mars Orbiter Camera on board the Mars Global Surveyor. In images taken during the end of the southern spring, when the surrounding surface is free of the seasonal frost, CO2 ice still appears to be present within the polygonal troughs. In the previous paper we described how the thickness of CO2 ice in the troughs evolves throughout the seasons. Here, we consider the influence of this cycle on the spatially averaged surface temperature, which can be determined from the orbital IR measurements. We model data from the Thermal Emission Spectrometer (TES) on Mars Global Surveyor to constrain the physical properties of the regolith in general and water ice content in particular.

We have found that the spring evolution of the spatially averaged surface temperature of the polygonal regions occurs in several distinctive steps. After recession of the seasonal ice cap, temperature increases rapidly no longer being controlled by the condensation point of the surface CO2 ice. During this first phase it reaches a value of about 20K lower than that expected for smooth surface. This effect is observed in TES data and is due to the fact that CO2 ice within the polygonal troughs is still present at this stage. The sublimation of CO2 ice within the troughs and the resultant increase of the surface temperature to the expected value of about 275K takes another 20 sols. The exact time when and in which way this CO2 ice sublimes completely away depends on several parameters. Our simulations suggest that the most important parameter is the thermal conductivity of the regolith which in turn depends on the amount of the subsurface water ice. Thus, measurements of the surface temperature of regions covered with polygonal features can be used to deduce the content of water ice in the regolith.